Card Details Popup, every "Sub-Popup" must use .back() instead of .close()

- .close() closes the popup. Until now it was ok, but as the Card
  Details are opened in a popup too, .back() must be used to get the
  same behaviour back
- .back() is also .close() if there is no "popup layer" left

client/lib/exportHTML.js

@@ -187,7 +187,7 @@ window.ExportHtml = Popup => {
     const boardSlug = getBoardSlug();
 
     await addJsonExportToZip(zip, boardSlug);
-    Popup.close();
+    Popup.back();
     closeSidebar();
     cleanBoardHtml();